With conventional semi-conductor memory components a distinction is made between so-called function memory components (i.e. PLAs, PALs, etc.), and so-called table memory components, i.e. ROM components (ROM=Read Only Memory), and/or non-volatile memories)—in particular PROMs, EPROMs, EEPROMs, flash memories, etc., and RAM-components (RAM=Random Access Memory and/or Read-write memories), for instance DRAMs and SRAMs.
A RAM component is a memory device in which data is stored under a specified address, from which address the data can later be read out again.
As a RAM component needs to accommodate as many memory cells as possible, it becomes important for this to be achieved as simply as possible.
With SRAMs (SRAM=Static Random Access Memory) for instance the individual memory cells consist of a few, for instance six transistors, and with so-called DRAMs (DRAM=Dynamic Random Access Memory) generally only of a single, appropriately controlled capacitive element (for instance a trench condenser), with the capacitance of which one bit at a time can be stored as a charge.
This charge only persists only for a very short time and therefore a so-called “refresh” needs to be performed, e.g., at an average of every 64 ms.
In contrast to this, no “refresh” needs to be performed on SRAMs; i.e. the data stored in the memory cell remains there for as long as a suitable supply voltage is supplied to the SRAM.
In contrast to this, the data stored in non-volatile memory components (NVMs and/or non-volatile memories), for instance EPROMs, EEPROMs and flash memories, remains intact even when the supply voltage is switched off.
Furthermore, so-called “resistive” and/or “resistive switching” memory components have—recently—also become known, for instance so-called phase change memories (Phase Change Memories), PMC memory (PMC=Programmable Metallization Cell), CB memories (CB=Conductive Bridging memories), etc.
With “resistive” and/or “resistive switching” memory components, an “active” material—for instance placed between two corresponding electrodes (i.e. a anode and a cathode)—is brought into a more or less conductive state by means of appropriate switching processes (more accurately: by means of appropriate current pulses of a corresponding height and duration). Here the more conductive state corresponds with a stored logic “one”, and for instance the less conductive state with a stored logic “zero”, or vice versa.
With phase change-memories (Phase Change Memories (PC memories)) the “active” material—connected between two corresponding electrodes—may for instance be a suitable chalcogen compound (for instance a Ge, Sb, Te or an Ag, In, Sb, Te compound).
The chalcogen compound material can be brought into an amorphous, i.e. a relatively poor conductive, or a crystalline, i.e. a relatively strong conductive state by means of appropriate switching processes (whereby for instance the relatively strong conductive state may be a stored logic “one”, and the relatively weak conductive state a stored logic “zero” or vice versa).
Phase-change memory cells are for instance known from G. Wicker, Nonvolatile, High Density, High Performance Phase Change Memory, SPIE Conference on Electronics and Structures for MEMS, Vol. 3891, Queensland, 2, 1999, as well as for instance from Y. N. Hwang et. al., Completely CMOS Compatible Phase Change Nonvolatile RAM Using NMOS Cell transistors, IEEE Proceedings of the Nonvolatile Semiconductor Memory Workshop, Monterey, 91, 2003, S. Lai et. al., OUM—A 180 nm nonvolatile memory cell element technology for stand alone and embedded applications, IEDM 2001, etc.
When programming an appropriate PMC memory cell in the case of a PMC memory (PMC=Programmable Metallization Cell)—depending on whether a logic “one”, or a logic “zero” is to be written into the cell—by means of appropriate current pulses of a corresponding height and duration, a corresponding metal “dendrite” (for instance of Ag, or Cu, etc.) is created by the electro-chemical reaction caused in an active material placed between two electrodes (which leads to a conductive state of the cell), or broken down (which leads to a non-conductive state of the cell).
PMC memory cells are for instance known from Y. Hirose, H. Hirose, J. Appl. Phys. 47, 2767 (1975), and for instance from M. N. Kozicki, M. Yun, L. Hilt, A. Singh, Electrochemical Society Proc., Vol. 99-13, (1999) 298, M. N. Kozicki, M. Yun, S. J. Yang, J. P. Aberouette, J. P. Bird, Superlattices and Microstructures, Vol. 27, No. 5/6 (2000) 485-488, as well as for instance from M. N. Kozicki, M. Mitkova, J. Zhu, M. Park, C. Gopalan, “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandary?”, Proc. VLSI (2002), and R. Neale: “Micron to look again at non-volatile amorphous memory”, Electronic Engineering Design (2002).
Furthermore so-called CB memories (CB=Conductive Bridging memories) are also known in state of the art technology.
CB memories are for instance described by Y. Hirose, H. Hirose, J. Appl. Phys. 47, 2767 (1975), T. Kawaguchi et. al., “Optical, electrical and structural properties of amorphous Ag—Ge—S and Ag—Ge—Se films and comparison of photo-induced and thermally induced phenomena of both systems”, J. Appl. Phys. 79 (12), 9096, 1996, as well as for instance by M. Kawasaki, et. al., “Ionic conductivity of Agx(GeSe3)1-x (0<x0.571) glasses”, Solid State Ionics 123, 1999, etc.
With CB memories, the switching process derives from the fact that when appropriate current pulses—of corresponding height and duration—are applied to an active material placed between two electrodes (for instance a suitable chalcogen (for instance GeSe, GeS, AgSe, CuS, etc.)) the elements of a corresponding precipitated “cluster” are made to increase in volume until the two electrodes are finally “bridged”, i.e. they become conductively interconnected (with the CB cell being brought into a conductive state).
By applying appropriate inverted current pulses, this process can be reversed again, whereby the corresponding CB cell can be brought into a non-conductive state again.
What is problematic about “resistive switching” is that in such memories the resistance between the electrodes in a particular state of the cell (i.e. either “conductive”, or “non-conductive”) may vary relatively strongly.
This variation makes it difficult to evaluate the above states by means of an appropriate evaluation circuit (i.e. it becomes difficult to determine whether a logic “zero”, or a logic “one” has previously been stored in the cell).